A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, and in more advanced devices, could be set remotely by a medical practitioner after implantation.
Early advances in pacemaker technology included the ability to sense intrinsic cardiac activity (i.e., the intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers," so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction (whether spontaneous or evoked). When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse.
Pacemakers such as those described above proved to be extremely beneficial in that they successfully reduced or eliminated seriously debilitating and potentially lethal effects of bradycardia in many patients. However, since pacemakers are implantable devices, a surgical procedure is required when pacing therapy is deemed appropriate by a medical practitioner. Further, many patients who receive pacemakers must expect to undergo several surgical procedures, because pacemakers have a limited life span, and therefore require replacement from time to time. Of course, it is always desirable to minimize the number of surgical procedures that must be performed on a patient to improve safety and reduce costs.
The life span of most pacemakers is dictated by the rate at which their batteries drain. Thus, a substantial effort has been directed toward minimizing the amount of energy used by pacemakers, while ensuring that the devices continue to deliver effective therapy. Demand pacemakers (discussed above) effectively reduce battery drain by delivering pacing pulses only when required. However, each pacing pulse delivered by a demand pacemaker may have a significantly higher energy content than that required to induce a cardiac contraction. Thus, even after the development of demand pacemakers, there remained an opportunity for further improvements in the area of pacemaker energy utilization.
The minimum amount of electrical stimulation that effectively evokes a cardiac contraction is commonly referred to as a patient's "capture threshold." Unfortunately, capture threshold varies significantly among patients; therefore, the amount of electrical stimulation provided by a pacemaker cannot be permanently set by the manufacturer. Rather, stimulus parameters must be individually set for each patient immediately after implantation and during subsequent office visits.
Determining a particular patient's capture threshold is a relatively simple procedure when performed during an office visit. Essentially, the medical practitioner can remotely adjust the amount of electrical stimulation downward from a maximum value that is known to elicit a contraction for all patients. Once the amount of electrical stimulation falls below the patient's capture threshold, a heart beat is not detected, and the medical practitioner then upwardly adjusts the amount of electrical stimulation beyond the last successful level.
Typically, a substantial safety margin is added to the measured capture threshold to ensure that the pacemaker continues to evoke contractions over an extended period of time. The safety margin is necessary because a patient's capture threshold varies over time--sometimes dramatically during the first few months following implantation. However, by adding such a large safety margin, it is almost assured that the pacemaker will be wasting significant amounts of energy during its life span.
In an effort to reduce the amount of energy wasted, pacemakers have been developed which automatically evaluate the patient's capture threshold during normal operation. These devices are also capable of automatically adjusting the amount of electrical stimulation in response to changes to the capture threshold. These features (which in combination are commonly referred to as "autocapture"), significantly reduce battery drain, because higher energy pacing pulses are delivered only when needed by the patient. Although most of these devices continue to add a safety margin to the measured capture threshold, the safety margin can be greatly reduced, especially when the capture threshold is measured frequently.
Prior art pacemakers which perform autocapture commonly use the patient's IEGM to determine when a pacing pulse has evoked a cardiac contraction. These prior art pacemakers sample the IEGM immediately after a pacing pulse is delivered. The shape of the IEGM waveform indicates whether the pacing pulse successfully captured the heart. If an evoked R-wave is detected in the IEGM soon after the pacing pulse, then capture is confirmed, and the pacemaker may then reduce the amount of electrical stimulation. If an evoked R-wave is not detected, then the pacemaker has reduced the amount of electrical stimulation too much. When this occurs, the pacemaker sets the amount of electrical stimulation back to the previous level that successfully evoked a contraction, and adds a safety margin.
Pacemakers that perform IEGM-based autocapture present several drawbacks, particularly relating to signal processing, which have proven difficult to overcome. For example, it is extremely difficult to accurately sense the IEGM immediately after a pacing pulse is delivered, due to the presence of residual electrical effects in the immediate vicinity of the pacing electrodes. These residual effects (commonly known as "afterpotentials") interfere with the pacemaker's ability to sense the IEGM. Indeed, most pacemakers enter a refractory period immediately after a pacing pulse is delivered, during which time the sensing circuitry is deactivated, for the specific purpose of avoiding undesirable sensing of afterpotentials.
Pacemakers which perform IEGM-based autocapture must therefore make certain accommodations to overcome the difficulties described above. For example, U.S. Pat. No. 3,757,792 describes a pacemaker which provides an IEGM-based autocapture feature, but which employs separate pacing and sensing leads. Interference from afterpotentials may be avoided if the sensing lead if it is placed sufficiently far from the pacing lead. However, the use of additional leads is undesirable, because it adds cost to the pacing system and complexity to the surgical procedure during which the pacemaker is implanted. In addition, the use of separate leads may limit the options available to the medical practitioner in configuring the system to meet the needs of a particular patient (e.g., lead selection, lead location, etc.).
If separate leads are not used, then highly specialized sensing circuitry is typically used to discern the IEGM over the afterpotentials. This approach is also undesirable, because the additional circuitry required to distinguish the IEGM from the afterpotentials may draw more current than would otherwise be the case, which offsets some of the energy savings achieved through implementing autocapture.
What is needed, therefore, is an improved pacemaker that automatically sets the amount of electrical stimulation provided by pacing pulses in accordance with a patient's capture threshold, without relying on the patient's IEGM. The improved pacemaker should perform autocapture without using leads that otherwise would not be necessary, and should not require processing circuitry that significantly increases battery drain.